Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images which show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions which emanate from the body and are captured by a scintillation crystal, with which the photons interact to produce flashes of light or “events.” Events are detected by an array of photodetectors, such as photomultiplier tubes, and their spatial locations or positions are calculated and stored. In this way, an image of the organ or tissue under study is created from detection of the distribution of the radioisotopes in the body.
One particular nuclear medicine imaging technique is known as Positron Emission Tomography, or PET. PET is used to produce images for diagnosing the biochemistry or physiology of a specific organ, tumor or other metabolically active site. Measurement of the tissue concentration of a positron emitting radionuclide is based on coincidence detection of the two gamma photons arising from positron annihilation. When a positron is annihilated by an electron, two 511 keV gamma photons are simultaneously produced and travel in approximately opposite directions. Gamma photons produced by an annihilation event can be detected by a pair of oppositely disposed radiation detectors capable of producing a signal in response to the interaction of the gamma photons with a scintillation crystal. Annihilation events are typically identified by a time coincidence between the detection of the two 511 keV gamma photons in the two oppositely disposed detectors, i.e., the gamma photon emissions are detected virtually simultaneously by each detector. When two oppositely disposed gamma photons each strike an oppositely disposed detector to produce a time coincidence event, they also identify a line of response, or LOR, along which the annihilation event has occurred.
On the other hand, Magnetic Resonance Imaging (MRI) is primarily used for obtaining high quality, high resolution anatomical and structural images of the body. MRI is based on the absorption and emission of energy in the radio frequency range primarily by the hydrogen nuclei of the atoms of the body and the spatial variations in the phase and frequency of the radio frequency energy being absorbed and emitted by the imaged object. The major components of an MRI imager include a cylindrical magnet, gradient coils within the magnet, an RF coil within the gradient coil, and an RF shield that prevents the high power RF pulses from radiating outside of the MR imager, and keeps extraneous RF signals from being detected by the imager. A patient is placed on a patient bed or table within the magnet and is surrounded by the gradient and RF coils.
The concept of merging PET and MR imaging modalities into a single device is generally known in the art. See, e.g., U.S. Pat. No. 4,939,464, incorporated herein by reference in its entirety. See also copending U.S. application Ser. No. 11/532,665 assigned to the same assignee herein. Recently, there has been increased interest and research in using this combined modality to provide accurate functional and structural quantitative images for applications such as diagnosis of stroke patients, oncology, brain mapping and Alzheimer's research.
In order to quantify the uptake of administered PET tracers by the investigated organs it is necessary to measure the amount of radioactivity in arterial blood. Consequently, an important device that is used for functional imaging with PET is an automated Blood Sampler—a device that measures the specific amount of radioactivity per blood volume in the artery over time. This information is used in the calculation of a 3D map of the metabolic rate in the observed region of the patient, test person or test animal.
Conventional blood samplers have the following basic design. A pump draws blood from an artery of the observed person or animal at a constant pump speed. This arterial blood is conveyed via a catheter through an arrangement of PET scintillator crystals (such as BOO, GSO or LSO). If positron decay occurs in the catheter, the emitted positron will annihilate with an electron in its direct proximity. Thus, two annihilation photons emerge with a definite energy of 511 keV. The PET scintillation crystals surround the catheter in such a way that it is very likely that both photons will be absorbed and translated into scintillation photons. The scintillation light is collected usually by photomultiplier tubes (PMTs) that are attached to the crystal.
For most applied scintillators, the energy of the absorbed particles is proportional to the amount of generated scintillation light. Suitable electronics distinguish between events which actually originate from the catheter within the field-of-view of the blood sampler and background events that may come from the patient or other radioactive sources. For example, this can be achieved by counting only events in which a total energy of around 1022 keV (=2×511 keV) is deposited in the crystals. Another method is coincidence detection. In this method, there are two optically separated crystals on opposite sides of the catheter. An event is counted if both crystals detect a 511 keV photon within a very short time of each other. The timing window is typically in the order of 10 ns.
Since commercially available automated blood sampling systems cannot be operated in the strong magnetic field of MR scanners, there exists a need in the art for a new blood sampler that is MR-compatible, such that an MR/PET multimodality imaging system may be used to its full potential.